In these days, the development of techniques for natural energy and the widespread use of these techniques are urgently needed for preventing global warming and for avoiding the depletion of limited fossil fuel resources. Among various types of natural energy, biomass energy is often regarded as the most promising natural energy from the viewpoint of its abundance and storability. Direct combustion of woody biomass which has hitherto been adopted, however, suffers from limited amount of resource and low efficiency, and, further, only electric power can be supplied by the direct combustion of woody biomass. On the other hand, in the production of ethanol by fermentation, only a limited part of biomass, for example, sugar such as sugarcane juice or molasses and starchy materials such as corn and potatoes can be used as the feedstock. The development of techniques, which can utilize the whole of biomass including cellulose and hemicellulose occupying a major part of photosynthetic products and can produce liquid fuels for engine or transportation applications, such as methanol, is of great significance. At the present time, however, such techniques are not in a practical stage for technical and economical reasons.
For example, Japanese Patent Laid-Open No. 138580/1995 describes an example of conventional techniques for gasifying organic matter such as biomass to obtain gas for use in the synthesis of methanol. This method includes partial oxidation of organic matter with air or oxygen and steam. In this case, the molar ratio of feed steam to carbon contained in the organic matter is regulated to 1 to 10, and the combustion or gasification temperature is regulated to 700 to 900° C. Japanese Patent Laid-Open No. 111254/1997 proposes a method wherein tar and soot, contained in gas produced by the above method, are decomposed in a gas decomposition furnace, into which a nickel-containing alloy or a nickel catalyst has been packed, provided on the downstream side of the gasification furnace.
On the other hand, Japanese Patent Laid-Open No. 41767/1995 proposes a method wherein biomass is decomposed within a vertical retort at 600 to 1200° C., generally 800 to 1100° C., in the presence of water.
According to the present inventor's finding, the above-described conventional methods respectively have the following problems to be solved.
(1) The Method in which Biomass is Partially Oxidized with Air or Oxygen and Steam
In this method, tar is produced as a by-product at a reaction temperature of 700° C. or below. On the other hand, when the reaction temperature is 900° C. or above, soot is produced. The tar and soot reach the interior of the gasification furnace and heat recovery equipment and gas scrubbing equipment provided on the downstream side of the gasification furnace, and further reach even a shift reactor, CO2 removing equipment, and methanol synthesizing equipment, resulting in blocking or lowered reaction yield. An attempt to maintain the temperature of the gasification furnace at 700° C. or above for avoiding this unfavorable phenomenon requires the combustion of a given part of the biomass. Consequently, the yield of the contemplated gas is lowered. Further, in this case, air or oxygen should be introduced into the gasification furnace for the combustion purposes. The use of air, however, results in dilution of the gas for methanol synthesis with nitrogen, and the use of oxygen requires the provision of a troublesome process for purifying pure oxygen from air. Therefore, this method is also disadvantageous in production cost.
(2) The Method in which a Gas Decomposition Furnace is Provided Downstream of the Gasification Furnace
It is considered that, in the course of travel of the decomposition product from the gasification furnace to the gas decomposition furnace, a polymerization or condensation reaction of the decomposition product proceeds. Further, this method is also disadvantageous in thermal efficiency, because, when the gas produced in the gasification furnace is led to the gas decomposition furnace, heating should be newly carried out. Furthermore, since the catalyst used in the gas decomposition furnace is severely deteriorated, frequent replacement of the catalyst is necessary.
(3) Decomposition in the Presence of Water
In this method, treatment at a high temperature of about 1000° C. is necessary. Further, this method is considered unsuitable for continuous treatment. In Japanese Patent Laid-Open No. 41767/1995, there is no description on the composition of the produced gas. However, for some composition, the degree of reliance on composition adjustment in steam reforming, which is carried out on the downstream side, is so large that the yield of methanol is inevitably lowered.